It shall be unlawful for the operator of a vehicle to stop, stand or park said vehicle for a period of time longer than is necessary for the expeditious loading or unloading of passengers in any place marked as a passenger zone. It shall be unlawful for the operator of vehicle to stop, stand or park said vehicle for a period of time longer than is necessary for the expeditious loading or unloading of passengers, or for the unloading and delivery of pick up and loading of materials, in any place marked as a loading zone. In no case shall the stop for loading and for unloading of materials exceed 30 minutes.